Optically variable security devices such as thin films, holograms, gratings, micro-prisms, photochromics, and more recently, microlens-based film structures (hereinafter collectively referred to as OVDs), are recognized as valued additions to secure documents such as banknotes. These devices allow for a variety of self-authenticating optical effects while rendering the secure document more resistant to counterfeiting.
Microlens-based OVDs are described in U.S. Patent Application Publication No. 2005/0180020 A1 to Steenblik et al. The film material or structure described in this reference employs a regular two-dimensional array of non-cylindrical lenses to enlarge micro-images and, in one embodiment, comprises (a) an optical spacer; (b) a regular periodic planar array of image icons positioned on one surface of the optical spacer; and (c) a regular periodic array of lenses positioned on an opposing surface of the optical spacer. The images projected by this film structure show a number of visual effects including orthoparallactic movement.
OVDs in the form of security patches, are mounted on one or both surfaces of a security document (e.g., banknote), while OVDs in the form of security strips or threads, are partially embedded within the document, with the OVDs being visible in one or more clearly defined windows on one or both surfaces of the document.
One of the primary requirements of banknotes and other secure documents is that the document must resist the effects of circulation. These documents must be durable (i.e., resistant to fold damage, tearing and soiling) and resistant to moisture and chemical absorption. In addition, the print which is applied to the document must adhere well, especially under severe conditions such as mechanical abrasion and accidental laundering.
In order to render banknotes and other secure documents more resistant to the effects of circulation; manufacturers and printers have coated the documents with certain varnishes and polymeric coatings. These varnishes and coatings, which consist of either ultraviolet (UV) radiation-crosslinkable prepolymers (100% solids), or resin mixtures with different host solvents (resin solids content ranging from 30 to 50% by weight), serve to seal the surface of the document increasing its resistance to soiling and moisture. Typically applied in a final, or near final step in the document's production using standard coating techniques (e.g., roller coating, gravure coating, air knife coating, roll coating, blade coating), these surface coatings are generally referred to as post-print varnishes. Coat weights applied to each side of the document surface range from 0.5 grams per square meter (g/m2) to 5.0 g/m2.
A more recent trend has been to apply a coating to substrates used in the production of these secure documents either during or immediately following manufacture. These surface coatings, commonly referred to as pre-print coatings, may be described as aqueous resin binder systems that serve to render the document resistant to moisture and soiling. Pre-print coatings may constitute or make up 1 to 15% of the document's finished mass.
Unfortunately, OVDs in secure documents subjected to one or both of these prior art techniques are at least partially obscured or otherwise adversely affected as a result of the overlying varnish or coating. As will be readily appreciated by those skilled in the art, OVDs rely on unique surface topographies in order to produce novel and specifically engineered visual and machine verifiable effects. Covering these surfaces with coatings and varnishes can cloud, mute, distort or otherwise diminish the features' effect.
As the requirement for resistance to soiling and moisture increases, generally the amount of pre-print coatings and/or post-print varnishes applied to the substrate is likewise increased. A trade-off then occurs in the form of increased substrate durability in exchange for reduced performance and effectiveness of some security features. In addition, some types of varnishes contain light scattering or light diffusing additives to reduce an appearance of glossiness on the finished, varnished documents. These additives can further decrease the effects of some security features.
In an effort to avoid these detrimental effects on the optically variable effects generated by OVDs, certain manufacturers (i) use very light coat weights of pre-print coatings or post-print varnishes, which reduces the document's ability to resist moisture and soiling, (ii) avoid the combination of pre-print coatings or post-print varnishes with certain OVD security features, or (iii) block the areas on the document surface prior to applying the pre-print coating or post-print varnish, which leaves significant areas of the document surface unprotected and unduly complicates the application process.
It has been discovered by the present inventors that the optical effect of these OVDs can be preserved without compromising soil and/or moisture resistance by applying a soil and/or moisture resistant formulation by way of a size press or other similar device instead of by way of standard coating techniques. It has also been discovered that thin layers of fibers (e.g., papermaking fibers) overlying and thus embedding portions of security devices in windowed secure documents rendered soil and/or moisture resistant in this way demonstrate increased durability.
The present invention therefore generally provides a method for imparting soil and/or moisture resistance to a porous substrate used in the manufacture of secure to documents, the porous substrate having a thickness. The inventive method comprises (a) applying a soil and/or moisture resistant formulation to opposing surfaces of the porous substrate, (b) forcing the soil and/or moisture resistant formulation into the pores of the substrate, the formulation thereby penetrating and extending throughout at least a portion of the thickness of the substrate, and (c) removing excess formulation from opposing surfaces of the substrate. Preferably, a size press (e.g., puddle or metering) or other similar device is used to force the soil and/or moisture resistant formulation into the pores of the substrate and to remove excess formulation from opposing surfaces thereof.
In a first contemplated embodiment, the inventive method imparts soil and/or moisture resistance to the porous substrate without obscuring optically variable effects generated by non-porous OVDs contained (or exposed) on a surface thereof, the method comprising:                (a) applying a soil and/or moisture resistant formulation to opposing surfaces of the porous substrate, the substrate supporting one or more non-porous OVDs; and        (b) employing a size press or other similar device to force the soil and/or moisture resistant formulation into the pores of the substrate and to remove excess formulation from opposing surfaces thereof, thereby leaving exposed surfaces of the non-porous OVDs substantially free of the soil and/or moisture resistant formulation.The term “non-porous OVDs”, as used herein, includes those OVDs having substantially or essentially non-porous surfaces, and those OVDs having surfaces that are substantially or essentially non-porous only in areas contained (or exposed) on a surface of the porous substrate.        
In a second contemplated embodiment, the inventive method imparts soil and/or moisture resistance to a windowed porous substrate supporting one or more security devices while increasing the durability of the substrate in areas overlying the security device(s), those areas of the substrate framing the device(s) and forming at least one window through which the security device(s) is exposed, the method comprising:                (a) applying a soil and/or moisture resistant formulation to opposing surfaces of the porous substrate having the one or more security devices partially embedded therein and visible in one or more windows on at least one surface thereof; and        (b) employing a size press or other similar device to force the soil and/or moisture resistant formulation into the pores of the porous substrate and to remove excess formulation from opposing surfaces thereof.        
The present invention also generally provides a soil and/or moisture resistant secure document, which comprises at least one porous substrate having a thickness, and an effective amount of a soil and/or moisture resistant formulation contained within the pores and on opposing surfaces of the porous substrate(s), wherein the soil and/or moisture resistant formulation is distributed throughout at least a portion of the thickness of the porous substrate(s).
In a first contemplated embodiment, the inventive soil and/or moisture resistant secure document further comprises one or more non-porous OVDs contained on and/or partially within the substrate(s), wherein the one or more non-porous OVDs have exposed surfaces that are substantially free of the soil and/or moisture resistant formulation. The phrase “substantially free”, as used herein, means that the non-porous OVDs have only residual or trace amounts of formulation on exposed surfaces thereof.
In a second contemplated embodiment, the inventive soil and/or moisture resistant secure document is a windowed secure document having one or more security devices partially embedded therein and exposed in one or more windows, those areas of the secure document overlying the one or more security devices demonstrating increased durability. Preferably, the one or more security devices are non-porous, optically variable, security strips or threads having surfaces that are substantially free of the soil and/or moisture resistant formulation.
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.